1. Field of the Invention
The invention relates to a process for preparation of an article made of enameled sheet.
2. Description of the Related Art
In order to enamel a sheet steel, there are deposited successively on the sheet a "mass" coat of enamel, then a so-called "glaze" coat of enamel.
The mass coat of enamel serves as a setting coat on the sheet.
The glaze coat is the finishing coat.
But a sheet also may be enameled without using a setting coat, therefore without a mass enamel: it is what commonly is called enameling "by a direct method."
In enameling by a direct method, as in any other enameling technique, several coats of glaze enamel also may be superposed.
In order to enamel a sheet by a direct method, by direct application of a glaze coat, the metallic surface to be enameled is prepared so as to obtain, in particular, a good adhesion of the enamel coat to the sheet.
In order to implement an article made of sheet steel enameled in this manner, such as a cooking pot or a casserole, from a cut sheet-steel strip, the following operations are undertaken in the following order: shaping, scouring, surface treatment, enamel-glaze frit coating and firing of the enamel.
Prior to shaping the sheet, it generally is advisable to apply a lubricating agent, for example a coat of oil, and following this operation, it then is advisable to clean the sheet (just prior to scouring).
The sheet which is used for shaping generally already is treated for temporary protection against corrosion (under transport and storage conditions); this surface protective treatment, applied at the conclusion of manufacture of the sheet strips, also may have a lubricating function.
In order to prepare the metallic surface for direct enameling, the metallic surface therefore first is scoured, then treated with a solution containing nickel as a coat-forming cation; this surface treatment also is called "nickeling."
Scouring and surface treatment generally are carried out by immersion or spraying.
In order to obtain a good adhesion and a good surface quality for the enamel, it generally is advisable to perform a deep scouring corresponding to a removal of material in excess of 15 g/m.sup.2 /face, usually on the order of 25 g/m.sup.2 /face of surface to be enameled.
This scouring is necessary to create a microrugosity on the surface, facilitating the adhesion of the enamel.
This deep scouring is a drawback to the extent that it generates very significant volumes of sludge to be treated.
As this scrubbing operation is to be carried out after the shaping, it generally is carried out at the enamelers' themselves, who generally do not have adequate installations for treatment of effluents.
The treatment of these scouring sludges therefore represents a very significant handicap.
The purpose of the invention is to mitigate this handicap by moving the scouring operation prior to shaping to the producer of sheet strips itself, which does have efficient and economical installations for treatment of sludges suited to a mass production and also devoted to the treatment of effluents from other operations, such as those of rolling of steel strips.
The document FR 2 196 400 describes a process for preparation of an article made of enameled sheet by a direct method which comprises the following operations in the following order:
with a view to obtaining a ground surface (to be enameled), the sheet steel is chemically attacked by electrolysis or, according to example 4, a chemical scouring with sulfuric acid is undertaken until removing 21.5 g/m.sup.2 of material; PA1 there is applied to the ground surface an aqueous solution of soluble glass so as to form, after drying, a temporary coat of soluble glass; PA1 optionally, a coat of wax is applied on the coat of soluble glass; PA1 it is shaped, in particular by stamping; PA1 the surface to be enameled is cleaned and washed with water; PA1 optionally, a nickeling treatment is performed; PA1 coating with an enamel-glaze frit is performed and the enamel is fired. PA1 the soluble glass is a material similar to the protective coat for the enameling and so the residues of soluble glass have no detrimental action on the coat for enameling; PA1 the coat of soluble glass, of 2 to 5 .mu.m in thickness, also is useful for the protection of the surface with a view to shaping; it makes it possible to improve appreciably the capacity for deep stamping; it therefore has a "prelubricating" function; PA1 the solutions of soluble glass themselves being used as a cleaning agent, the coat of soluble glass is easily cleanable, in particular following shaping; PA1 the coat of soluble glass affords a protection against corrosion provided it is itself covered with a coat impervious to moisture, such as the coat of wax cited as an option or such as a protective coat of lubricant (oil). PA1 a scouring operation followed immediately by a single surface-treatment operation for temporary protection against corrosion and for prelubrication, PA1 a shaping operation followed by a cleaning, PA1 a so-called nickeling surface-treatment operation, PA1 and an operation for direct enameling, properly speaking, PA1 and characterized in that said surface treatment for temporary protection against corrosion and for prelubrication is selected so as to meet the following three criteria, when it is applied after scouring on a bare sheet steel with a thickness of 1 mm and gradation of DC 03 ED or DC 04 ED by reference to EN standard 10209, published in Nov. 1996: PA1 the so-called temporary corrosion-protection and prelubrication surface treatment is carried out by application of a uniform coat of oil, by application of a uniform coat of aqueous oil emulsion or by application of an aqueous solution containing, in solution, a fatty alcohol ethoxylate and a corrosion inhibitor. PA1 the conditions for nickeling are set in order to obtain a deposit ranging between 0.5 and 2.5 g/m.sup.2 on the surface of the article to be enameled. PA1 the period separating the scouring and surface-treatment operation and the shaping operation is in excess of 1 day.
According to this document:
According to the process described in this document FR 2 196 400, it therefore is seen that in order to stock the scoured sheets prior to shaping without risk of corrosion, while taking advantage of a prelubrication treatment, it is advisable to apply at least two coats on the scoured sheet: a coat of soluble glass and a coat of wax.
The application of these two coats requires two successive treatments.
The present invention also has the purpose of applying, in a single treatment, an easily cleanable coat for temporary protection against corrosion and for prelubrication.